1. FIELD OF THE INVENTION
The invention relates to a device for measuring the distance to close and far-off objects by which laser beams modulated and emitted by the device are reflected, according to the preamble of claims 1 and 4.
2. Description of the Background Art
Such devices for measuring the distance to an object with an accuracy within a few millimeters have long been known and are now used in large numbers for a very wide range of applications, in particular in surveying and construction. For measuring the distance, a transmitted bundle of optical rays are projected by such a device toward the object to be measured. At least some of the rays of the transmitted bundle are reflected by the object. Rays which comprise rays reflected by the object and interfering background rays are collected by means of an optical system of the device and are converted by a receiver of the device into a single electrical signal which comprises a useful signal which can be coordinated with the respective rays or background noise. On the basis of the propagation velocity of optical rays, the distance to the object can be determined by evaluating the electrical signal.
Depending on the respective application and the required distance range, measurement is effected to different types of objects.
On the one hand, measurements of the distance in a range from a few centimeters to a few hundred meters can be carried out to an object which reflects the transmitted rays unoriented and scattered back to the device. Such an object typically has a naturally rough surface. The distance is determined by that region of the surface which the transmitted bundle strikes. The size of the illuminated region thus determines the local resolution of the measurement on the surface of the object which reflects with scattering. In the case of many objects, a high local resolution is a pre-condition for an accurate measurement. The smaller the divergence of the transmitted bundle, the smaller is the illuminated region and—according to DE 198 40 049 A1—the higher in general is the fundamentally achievable accuracy of the measurement of the distance to an object which reflects with scattering.
On the other hand, distance measurements in a wide range from a few centimeters to several kilometers can be carried out to an object which reflects the emitted rays oriented back to the device. An object which reflects in an oriented manner is in practice often in the form of a corner cube prism of ground glass. Since such an object has retroreflectivity which is several orders of magnitude greater than an object which reflects with scattering, only the object reflecting with orientation can be selectively measured by the device. Automatic target seeking devices of many surveying instruments are now also based on this property. It is known that the greater the divergence of the transmitted bundle, the less exactly need a device for distance measurement be aligned with such an object. Without exact alignment of the device, it is possible—according to DE 198 40 049 A1—considerably to increase the convenience of the distance measurement to an object which reflects with orientation, in particular in the case of short and medium distances.
In such devices, two different basic forms of the arrangement of the transmitted bundle relative to the received bundle are known. In the monoaxial basic form, a common objective is provided for the received bundle and the transmitted bundle. Today, the transmitted bundle is as a rule arranged inside the received bundle with the aid of a central reflecting element. For this purpose, the central reflecting element is as a rule arranged on the optical axis of the common objective. In the biaxial basic form, on the other hand, the received bundle and the transmitted bundle are arranged a distance apart. In each case a separate objective is provided for the transmitted bundle and the received bundle.
Compared with the respective other basic form, the two basic forms have advantages and disadvantages which are of different importance depending on use.
Independently of the basic form, however, a problem with the detection of the transmitted rays reflected by the object occurs in the case of such devices on measurement to an object unless special measures are taken, since—independently of the basic form—from a certain distance the reflected transmitted rays to be converted by the receiver become fewer the closer the object is to the device. If the useful signal coordinated with the reflected transmitted rays falls below a critical strength, the accuracy or the rapidity of the measurement is impaired or a measurement even becomes completely impossible.
In the case of a device of monoaxial basic form, a problem with detection may occur owing to shadowing due to the central reflecting element during measurement to a close object. The reflecting element blocks the path to the receiver for at least a part of the reflected transmitted rays collected by the objective. The closer the object to be measured is to the device, the greater the extent to which the collected transmitted rays migrate in the focal plane of the objective away from the centre toward the periphery and, depending on the formation of the receiver, may be lost with regard to conversion into an electrical signal. Both during measurements to a close corner cube prism which reflects in an oriented manner and with whose corner the transmitted bundle is centrally aligned and during measurements to a close object which reflects with scattering, the useful signal can fall below a critical value. Various measures are known for avoiding failure to reach said critical values.
DE 101 28 484 A1 discloses a distance-measuring device which has a common objective, a transmitted bundle in the form of a collimated laser beam and a received bundle arranged coaxially thereto. A measure for overcoming the shadowing problem provides three optical waveguides which are arranged side by side and whose entry areas of different sizes are arranged side by side in the focal plane of the object. The largest entry area is coordinated with measurements to far-off objects. The received bundles transmitted by the optical waveguides are focused via a collecting lens onto a detector. The received bundle is expanded by the three entry areas so that even distances to close and far-off objects can be measured. An alternative measure provides a single optical waveguide having an entry area of the same dimensions. Since in general the circumference of interfering rays of the background is also proportional to the entry area of the optical waveguide, an optical screening mask is arranged in front in order to improve the ratio of reflected transmitted rays and interfering rays with the background. The mask has a central diaphragm and at least one lateral diaphragm whose diameter is small in comparison with the central diaphragm.
DE 696 08 066 T2 discloses a distance-measuring device which has a common objective, a transmitted bundle in the form of a collimated laser beam and a received bundle arranged coaxially therewith. For measuring short distances to objects which reflect with scattering, a deflecting device is provided which deflects measured light which is reflected with scattering and has migrated from the optical axis of the objective back toward the axis. The provision of such deflecting devices gives rise to corresponding costs in the production of such devices.
In the case of a device of biaxial basic form, problems occur with detection in the case of measurements to a close object, owing to a parallax of the transmitted and received bundles which are a distance apart. The closer the object to be measured is to the device, the greater the extent to which the received bundle migrates in the focal plane away from the optical axis of the transmitted bundle a distance away. Both in the case of measurements to a close corner cube prism which reflects in an oriented manner and with whose corner the transmitted bundle is centrally aligned and in the case of measurements to a close object which reflects with scattering, the useful signal may fall below a critical value. The prior art discloses a multiplicity of very different measures to prevent said signal from falling below said critical value.
WO 03/002 939 A1 discloses a device for optical distance measurement of biaxial basic form comprising a transmitted bundle in the form of a collimated laser beam and an optical detector whose photosensitive area is extended in the shape of a drop or wedge, so that it is adequate for a beam shift of transmitted rays reflected by a target object even in the case of decreasing distances to the target object. The area tapers in the direction of the beam shift of decreasing target object distances.
DE 43 16 348 A1 discloses a device for distance measurement of biaxial basic form comprising an optical fiber which can be moved in the focal plane and which tracks the received bundle of the device as a function of the distance to the object to be measured. This measure firstly reduces the speed of the measurement and secondly requires a complicated design.
DE 692 25 811 T2 discloses an optical radar device for vehicles, whose transmitting device also provides a decollimated transmitted bundle of laser beams by means of a semiconductor laser diode and a multi-focal optical system and thus illuminates a carriageway section for optical distance determination to obstacles. With such a device, short to medium distances can be determined accurately to a decimeter.